I am thinking of painting the frame of my bike as it has a surprising amount of small rust speckles.

However last year I did a trailer and it was a disaster! I had stripped it back (sanded and wirebrushed) to bare metal where rusty, cleaned and sanded everywhere else, then I bought paint, a three paint system of rust proofer, undercoat and topcoat. Well despite being the same brand and very clean bits of it bubbled so I sanded them back again and re-primed and topcoated.

I also tried chassis/underbody matt black over a proprietary rust proofer on a separate area, the towbar. This was OK for a few months but rust is showing through again on all areas!

So what is the best rust-proofing and top-coating system you guys use? I don't have a sprayer and can't afford sandblasting or hot gal or such fancy stuff. I guess therefore I prefer a brush on method. I would prefer a matt black.